A Review and Evaluation of Instrumental Methods of Testing Phosphate Coatings

A critical review of instrumental methods of testing phosphate coatings based on the literature and on experimental work was carried out taking into account their utility for research and for routine quality control. The possible methods found in the review account for the relatively small knowledge of the corrosion phenomena that occurs at the phosphated metal/electrolyte phase boundary. Potential drop during phosphating is an interesting method of recording the process, but no theoretical explanation of the curve is given. So far, applicability of the method is limited to porosity assessments in certain phosphating solutions and to some practical checks, that is, activators before treatment. The voltammetric method seems to be very doubtful from the theoretical point of view since constancy of the surface area and stability of the coating before and during measurements cannot be ensured, and the mechanisms of the electrode processes in pores and on the unphosphated substrate were not proved identical. It is assumed that the impedance method will find wider application provided the reliability of assessment of elements of the equivalent circuit and automatization of measurements and calculation are improved.

The less doubtful methods are microscopic and X-ray diffraction examinations, particularly determining phosphophyllite to hopeite ratio. However, some complimentary examinations explaining the effect of sample geometry and texture and the relation between the line intensity and unit mass are required. At present, this is probably the only standardizable instrumental method of testing phosphate coatings.